Molecular basis for proton-dependent anion binding by soybean leghaemoglobin a

Abstract

HAEM proteins are widespread in nature and exhibit diverse biological activities. The properties and function of a given haem protein depend critically on the environment of the haem group. The reactivity of the haem is often influenced by one or more haem-linked ionisable groups. Thus, oxygen affinities of mammalian haemoglobins, for example, exhibit considerable pH dependence (the Bohr effect1). Anion binding to some haem proteins is also a proton-dependent reaction (for example, peroxidase2, catalase2 and leghaemoglobin3,4) whereas for others (myoglobin, haemoglobin1) it is essentially independent of pH. Identification of the ionisable groups which influence the ligand-binding reactions of haem proteins is of fundamental importance in the elucidation of structure/function relationships. Nuclear magnetic resonance (NMR) spectroscopy is one of the most direct methods for observation of titrating groups in proteins. We report here on NMR studies of the nicotinate complex of soybean ferric leghaemoglobin a. On the basis of these studies we propose a role for a haem propionic acid group as a mediator of anion binding.